The invention relates to a magnetically anisotropic recording medium for perpendicular (vertical) magnetization including a disc-shaped substrate of nonmagnetic material which is provided on at least one flat side with a lower layer, the material of which contains, at least as the main component, silicon, germaniun and/or titanium and to which a colbalt (Co)-containing alloy is applied in layers having an axis of easy magnetization which lies at least largely in the direction of the normal relative to the flat side of the substrate. Such a recording medium is illustrated in the publication "IEEE Trans. Magn.", Vol. MAG-21, No. 5, September '85, pages 1426 to 1428.
The principle of vertical magnetization for storing information in suitable recording media is generally known (see, for example, "IEEE Trans. Magn.", Vol. MAG-16, No. 1, Jan. '80, pages 71 to 76, and Vol. MAG-20, No. 5, September '84, pages 657 to 662 and 675 and 680). The recording media provided for this principle, which is often also called vertical magnetization, can be present, for instance, in the form of rigid magnetic storage discs. Such a recording medium has a recording layer which can be magnetized and comprises a material with vertical magneto-crystalline anisotropy, where the axis of the so-called easy magnetization of this layer is oriented perpendicularly to the surface of the recording medium. A preferred corresponding storage material is CoCr (see, for example, "IEEE Trans. Magn.", Vol, MAG-14, No. 5, September '78, pages 849 to 851). By means of special magnetic heads, the individual pieces of information can then be written as bits along a track in subsequent sections by suitable mangetization of the recording layer. The bits have here a predetermined dimension in the lengthwise direction of the track, also called wavelength. This dimension can be substantially smaller as compared to the limit which is given for storage according to the known principle of longitudinal (horizontal) magnetization. Thus, the information which can be stored in the special recording media can be increased by use of the principle of vertical magnetization.
For recording by vertical magnetization of such a medium, special magnetic heads have been developed of which the magnetic conductors formed by magnet legs have, in particular, a shape similar to a ring head (see, for example, European Pat. No. A-0 012 910). With this type of head, the problem arises however, to generate a sufficiently strong signal in the recording medium. This is due, among other things to the chosen material, CoCr, itself. As is well known, the signal level is determined by the magnitude of the coercivity of the material coupled to the magnetic remanence via the demagnetization factor (see, for example, "IEEE Trans. Magn." Vol MAG-18, No. 2, March '82, pages 769 to 771). Now, the remanence of CoCr is approximately equal to the coercivity since for this material, the demagnetization factor is nearly 1. While the coercivity can be increased within certain limits this must always be smaller than the magnetic writing field to be generated by the magnetic head for unequivocal magnetization conditions. However, because the desired miniaturization of the head design, correspondingly narrow limits are set to the magnitude of this head field. Thus, the maximum vertical head field intensities are generally below 100 kA/m.
One now attempts to solve the problem of insufficient head field strengths, for instance, by going to a type of head which has only a single magnet pole for writing (see, for example, "IEEE Trans. Magn." Vol. MAG-18, No. 6, November '82, pages 1170 to 1172 and European Patent No. A-0 071 489). The writing field of such a magnetic head, also called a single-pole head, can be obtained approximately also by special measures regarding the preference of one of the two magnet legs of a magnetic head with a shape similar to a ring head (see, for example, European Pat. No. A-0 166 818). In the mentioned head types, there is, however, the difficulty of returning the magnetic flux. One is therefore generally compelled to arrange under the recording layer of the CoCr alloy a further magnetically soft layer such as, for instance, a layer of "Permalloy" (Trade Mark of "Bell Telephone Manufacturing Co."). While such a lower layer leads advantageously, in the writing process, to an increase of the magnetic field strength at the point of the recording medium and thus, to the signal level, additional problems are connected with this measure such as a so-called "peak" or "bit shift" (see, for example, "IEEE Trans. Magn." Vol. MAG-19, No. 5, September '83, pages 1617 to 1619). These problems, however, have not been solved satisfactorily to date.
Such a magnetically soft lower layer is not provided in the recording medium which is known from the publication mentioned at the outset or European Pat. No. A-0 158 338. This medium contains a disc-shaped substrate of a nonmagnetic material such as aluminum. To this substrate is applied a lower layer of silicon or germanium with a thickness between 1 nm and 1 .mu.m. This layer serves as an underlayer for a CoCr layer to be applied thereon which should have a thickness in the order of 500 nm or more. The lower layer has, in particular, the purpose of promoting the incorporation of a vertical magnetic anisotropy in the CoCr layer to be deposited thereon. It has been observed that on such a lower layer, a CoCr material is formed, the hexagonal axis of which is perpendicular to the layer plane, where the hexagonal axis determines the easy direction of magnetization. It has been found however, that the magnetic remanence of such a recording layer and thereby, the signal level that can be obtained are distinctly smaller than in a CoCr layer with a magnetically soft lower layer.